Is null unique in PostgreSQL?

Question

While the SQL standard allows multiple nulls in a unique column, and that is how Postgres behaves, some database systems (e.g. MS SQL) allow only a single null in such cases. 7 нояб. 2019 г.

Can a NULL field be unique?

Regardless of using unique constraint or unique index, the field can accept null values, however the uniqueness will result in only accepting a single row with null value.

Does unique index allow NULL Postgres?

When an index is declared unique, multiple table rows with equal indexed values are not allowed. Null values are not considered equal. A multicolumn unique index will only reject cases where all indexed columns are equal in multiple rows.

IS NULL value allowed in unique key?

PRIMARY KEY constraint differs from the UNIQUE constraint in that; you can create multiple UNIQUE constraints in a table, with the ability to define only one SQL PRIMARY KEY per each table. Another difference is that the UNIQUE constraint allows for one NULL value, but the PRIMARY KEY does not allow NULL values.

IS NULL equivalent in PostgreSQL?

PostgreSQL IS NOT NULL operator.

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